Ancient Sumer question

[sksvlad]

In the following article (open the link) https://www.yahoo.com/news/ancient-sumerians-invented-water-flumes-195406238.html there is a passage closer to the end which says the following:
“We have not seen any other example of this in ancient Mesopotamia. If you look at the walls of the bridge, they are inclined outwards. This further increases flow, but no one knew this, as far as we know, until the 20th century.”
I do not understand how the walls of the bridge itself influence water flow. The walls supporting the bridge at the bank point WILL increase the flow, but that's not what the article says.

 

[HAZORD]

If I understand the article correctly, they necked down the path of the water, which would increase its speed which would increase the flow, if you are talking about velocity. The outward increasing wall would act like a funnel, channeling the wider path into a smaller orifice, causing the faster flow rate.